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Today, AP Physics 2 students used their computational models for projectile motion of a fluid stream to predict where on the floor to place their cup to catch the water from their 2L bottle with a hole in the side. When I planned this lab practicum over the summer, I was disappointed that we would be developing a computational model for a problem that is fairly easy to solve algebraically. Last year, we used a computational model for a lab practicum where the algebraic solution was beyond most students. I recently remembered advice given to me from a physicist at Fermilab: students must understanding that computational models must be verified against known outcomes before they are used to calculate unknown outcomes. I emphasized this point and encouraged students to check their computational model with an algebraic solution solved by hand. While I mentioned this yesterday, I bring it up again since, today, a student remarked that the computational model was off by 5 cm. He and I were both stumped by this since the projectile motion part of the model is fairly straightforward. Thankfully, another student found the bug. The origin of the model was set to the bottom center of the bottle. The water droplet exiting the hole started a displacement of half the bottle’s width from the origin. The bottle’s width was set to 10 cm. The reported position of the water droplet hitting the floor was based on the origin being located at the center of the bottle and not at the starting position of the water droplet. The student that found the bug quickly fixed it by setting the width of the bottle and enclosed column of water to 0 cm! While totally unplanned, this nicely emphasized the importance of verifying one’s computational model! I have since fixed my model by positioning the initial x position of the water droplet at the origin.

I was so busy today running around watching students demonstrate their prediction and passing out towels, that I forgot to take photos. Thankfully, @anna_kraftson was observing my class and did!